WEEK 7 DC SWITCHING POWER SUPPLIES, PART II 1

EXPECTATIONS Describe the supply isolation characteristics afforded by transformers. Draw basic forward, flyback and isolated Cuk topology schematics. Determine minimum inductances required in isolated switching supplies. Explain the practical tradeoffs between types of isolated converters. Describe the operating characteristics of resonant switching converters. Draw the sine wave characteristics of a resonant switch arrangement. List the three specific modes of load- resonant converters. Compare tradeoffs in switching topologies by considering loading conditions. 2

Ferromagnetism

There are many applications of ferromagnetic materials, such as the electromagnet. There are many applications of ferromagnetic materials, such as the electromagnet.applications electromagnetapplications electromagnet Ferromagnets will tend to stay magnetized to some extent after being subjected to an external magnetic field. Ferromagnets will tend to stay magnetized to some extent after being subjected to an external magnetic field. This tendency to "remember their magnetic history" is called hysteresis. This tendency to "remember their magnetic history" is called hysteresis. hysteresis The fraction of the saturation magnetization which is retained when the driving field is removed is called the remanence of the material, and is an important factor in permanent magnets. The fraction of the saturation magnetization which is retained when the driving field is removed is called the remanence of the material, and is an important factor in permanent magnets.remanence

Ferromagnetism Hysteresis Hysteresis

Ferromagnetic Materials MaterialTreatment Initial Relative Permeability Maximum Relative Permeability Iron, 99.8% pureAnnealed Iron, 99.95% pure Annealed in hydrogen 10,000200, Permalloy Annealed, quenched 8,000100,000 Superpermalloy Annealed in hydrogen, controlled cooling 100,0001,000,000 Cobalt, 99% pureAnnealed70250 Nickel, 99% pureAnnealed Steel, 0.9% CQuenched50100 Steel, 30% CoQuenched... Alnico 5 Cooled in magnetic field 4... SilmanalBaked... Iron, fine powderPressed...

Equivalent circuits of a transformer: (a) ideal transformer (b) transformer with the magnetizing inductance included 7

Coupled Inductor 8

A coupling factor, K, can be used to take care of the leakage inductance K=1 is often good enough for a simulation; "good" real transformers often have a K very close to 1 (e.g. K = 0.995). For power converters (e.g. flyback), however, it's good to use leakage inductances. N1/N2 = K * sqrt(L1/L2) 9

Forward converter 10

Flyback Converter 11

12

Midpoint Rectifier 13

Push-pull Converter 14

Half-bridge Converter 15

Full-bridge Converter 16

Voltage-mode resonant switches: (a) L-type half-wave (b) M-type half-wave (c) L-type full-wave (d) M-type full-wave 17

Current-mode resonant switches: (a) L-type half-wave (b) M-type half-wave (c) L-type full-wave (d) M-type full-wave 18

Waveforms of the inductor current and capacitor voltage in an undamped resonant circuit 19

Series-loaded resonant converter 20

AC equivalent circuit of the series-loaded resonant converter 21

Control characteristic of the series-loaded resonant converter 22

Parallel-loaded resonant converter 23

AC equivalent circuit of the parallel-loaded resonant converter 24

Control characteristic of the parallel-loaded resonant converter 25

Series-parallel resonant converter 26

Control characteristic of the series-parallel resonant converter 27

Soft-Switching DC-DC Converters Chapter 828 Is to shape the voltage or the current waveform by creating a resonant condition to: Force the voltage across the switching device to drop to zero before turning it ON  Zero-Voltage Switching (ZVS) Force the current through the switching device to drop to zero before turning it OFF  Zero-Current Switching (ZCS)

Hard-Switching and Soft- Switching Hard-Switching Zero-Current Switching Zero-Voltage Switching

Why Soft-Switching? Reduce switching losses especially at high switching frequencies Increase the power density, since the size and weight of the magnetic components is decreased by increasing the operating frequency Reduce the Electromagnetic Interference (EMI)

ZVS Converter 31

Quasi-resonant ZVS buck converter with L-type half- wave switch 32 Zero Voltage Switching (ZVS)

ZVS ADVANTAGES: The ZVS enables high frequency operation with high efficiency. Zero power “Lossless” switching transitions Reduced EMI / RFI at transitions No power loss due to discharging Goss No higher peak currents, (ie. ZCS) same as square wave systems High efficiency with high voltage inputs at any frequency Can incorporate parasitic circuit and component L & C 33

ZCS Eliminates the voltage and current overlap by forcing the switch current to zero before the switch voltage rises. For high efficiency power conversion, the ZCS topologies are most frequently adopted. 34

Quasi-resonant ZCS buck converter with an L-type full- wave switch 35 Zero Current Switching (ZCS)

Quasi-resonant ZCS boost converter with M-type full- wave switch 36 Zero Current Switching (ZCS)

The full-wave ZCS quasi- resonant switch cell Half wave Full wave

38

ZCS ADVANTAGES: ZCS technology for use in the charging test of a lead-acid battery, to demonstrate the effectiveness of the developed methodology. These techniques lead to either zero voltage or zero current during switching transition, significantly decreasing the switching losses. This increases the reliability for the battery chargers high quality, small size, light. The circuit structure is simpler and much cheaper. 39

Next Week Unit 8 Chapter 5 AC – AC CONVERTERS AC-TO-AC POWER CONVERSION 40